Three distinct classes of single-cell clones were derived from the B16 mouse melanoma: (1)\"null" clones produce tumors in syngeneic C57BL/6 mice but form few or no lung colonies following injection into the tail vein and fail to metastasize; (2)\colonizing clones form numerous lung colonies following intravenous injection but do not metastasize significantly; and (3)\metastatic clones spontaneously metastasize to the lungs but are inefficient at forming lung colonies when injected in the tail vein. Utilizing representative clones, the basis for distinction between tumorigenicity, lung colonization and lung metastasis will be probed, to provide novel information about the occurrence and apparent specific organ tropism of spontaneous metastasis. The capacity of tumor cells growing subcutaneously to invade into, and circulate within, blood vessels will be monitored by vascular perfusion and characterization of collected cells, and by histopathological examination. The ability of cells from subcutaneous growths or intravenously injected cells to arrest, extravasate and proliferate in organ parenchyma will be assessed similarly, and by measuring survival kinetics of radiolabeled cells, interaction of cells with host lymphocytes and platelets, invasiveness through chick chorioallantoic membrane, shedding of immunogenic surface molecules and specificity of organ tropism (homing to ectopically implanted organ fragments). The role of unique cell surface antigens related to murine leukemia virus gp70 (B16-gp70, B16-gp80 and B16-gp85) and of host immunity to these antigens in metastasis and colonization, similarly will be investigated to determine how specific host immunity can influence metastasis. Metastatic activity has been generated from null clones in vitro and in vivo, and lung metastases apparently result from nonspecific trapping. The ultimate goal of this research is to provide information to permit the designing of successful immunotherapy of metastatic tumors.